The development of equations relating the sedimentation coefficient to DNA molecular weight are generally based on moleculs no larger than the DNA of T-even bacteriophage. These equations are frequently extrapolated to molecular weights beyond this maximum, although the value of this practice is questionable. One of the reasons for the uncertainty is speed dependence (Zimm, 1974), an important factor in the measurement of S for large molecules. Furthermore, current speed-dependence theory does not describe the sedimentation behavior of DNA in the 1 to 2 times 10 to the 8th power molecular weight (Mr) range, while an alternate (incorrect) form of the theoretical equation does. This research intends to approach these problems by developing molecular weight standards in the 200 million to 500 million range using DNA from giant heads of phage T4 or other sources. These molecules will be used to determine whether the correct, incorrect or some other form of the speed-dependence equation applies at these molecular weights. Mr values will be determined by electron microscopy and viscoelastometry, and sedimentation coefficients will be determined by sucrose gradient sedimentation. The sedimentation studies will extend the basis of S vs. Mr equations beyond the limit imposed by the mass of DNA from normal T-even phage.